My personal opinion is that if you're using an ontology language, every
term you use must be defined in some ontology (even if only to say that
it is a class or property). Now many, including members of the Joint
Committee and the WebOnt WG, disagree with me. They often argue by
analogy to the Web dropping the need for all hypertext links to actually
point to something. This allowed the Web to scale beyond previous
hypertext systems by simply introducing 404 errors. However, I don't
think the analogy quite works here. The whole point of an ontology is to
achieve some standard agreement on the meaning of terms. If you can just
create a term out of the blue, then what's the point of having an
ontology in the first place?
Still, as you point out, somewhere the system will have to bottom out in
terms that do not have formal definitions. In description logics, they
call these primitive classes. The Semantic Web will have to have
primitive (undefined) classes (or else solve the symbol grounding
problem (ha!)), but I think that these should come from a set of terms
defined by some community (in an ontology). Now, it is not necessary
that everyone build their definitions from the same set of of primitive
classes (that wouldn't scale!), but it is important that when two
different domains need to be integrated, that it is possible to create
ontologies containing axioms that bridge (at least partially) between
the classes defined by the ontologies for these domains.
Jeff
Dan Brickley wrote:
>
> On Tue, 23 Apr 2002, Jeff Heflin wrote:
>
> > Tim,
> >
> > If you refer to a term in an ontology without importing then I suggest
> > that you are using a name without agreeing to its definition. This is
> > not an error condition, but it may mean that DAML+OIL-compliant agents
> > won't conclude everything you intend them to.
>
> This is going to happen a *lot*. Many terms (for some sense of that word)
> in xml/rdf won't be classes and properties, just plain old URI references.
>
> Is using a class/property name without 'agreeing to its definition' any
> worse than using some abitrary URI without similar agreement? (Can't we
> all just muddle along...?).
>
> Where does this stop? For example, consider a class
> http://example.com/xmlns/Bachelor defined as an unmarried man. Presumably
> to agree to the definition of 'Bachelor', we'd need to use the terms
> 'unmarried' and 'man'. And using those without agreeing to their
> definition would be... bad? So we'd need to agree to the definitions of
> all the terms used in their defintions... and so on.
>
> Dan
>
> --
> mailto:danbri@w3.org
> http://www.w3.org/People/DanBri/